The present invention relates to a process for the preparation of salts of 1-substituted 2,4-diaminobenzenes by catalytic hydrogenation of the corresponding 1-substituted 2,4-dinitrobenzenes and conversion of the resulting reaction product, following removal of the catalyst, into the salt form.
Salts of 1-substituted 2,4-diaminobenzenes are used in many different areas. Their synthesis is therefore of particular importance. 2-(2',4'-diamino-diaminophenoxy)-ethanol and its salts are used, for example, according to DE-A-27 58 735 and DE-A-27 37 138, in oxidation dye compositions as the meta-component.
DE-A-27 58 735 describes the preparation of 2-(2',4'-diaminophenoxy)-ethanol by catalytic hydrogenation of 2-(2',4'-dinitrophenoxy)-ethanol. To remove the catalyst, the reaction mixture after the hydrogenation is filtered in the warmed state, and the filtrate is recovered. To prepare the dihydrochloride salt of 2-(2',4'-diaminophenoxy)-ethanol, an excess of hydrogen chloride gas is then introduced into the filtrate. However, this method produces a yield of only 78%. Reworking this process also shows that the product is produced in a deep violet color and contains considerable amounts of by-products and decomposition products, which is manifested in a purity of only 86%. Measurement of the degrees of transmission of a 0.1% strength solution of the product in water according to DIN 55945 gives the results T.sub.x =38.6, T.sub.y =26.4 and T.sub.z =30.9.
Particularly when the salts of the 1-substituted 2,4-diaminobenzenes, such as 2-(2',4'-diaminophenoxy)-ethanol, are used in the cosmetics sector, e.g., as precursor compounds in the field of hair cosmetics, the purity of the 1-substituted 2,4-diaminobenzenes is a decisive factor: even slight contamination by foreign substances is problematical for medicinal reasons (e.g., danger of triggering allergies).
The object of the present invention was therefore to provide a process with which salts of 1-substituted 2,4-diaminobenzenes and in particular of 2-(2',4'-diaminophenoxy)-ethanol can be obtained with low impurities and low undesired discoloration.